Below you see all the terms used in defining exactly the features of which a cannon's cascabel is composed:
Cascabel knob
Cascabel neck
Breech face and moldings
Reference Only: Breech ring
Basically the cannon tube feature we call the cascabel is everything you find behind the rear edge of the Base Ring or Breech Ring which is the largest diameter of the tube located directly to the rear of the vent on almost all cannon, ( there are some exceptions where the vent intersects the breech face and mouldings). The Cascabel Neck is the smallest dia. feature within the whole Cascabel.
Also, Cannon builders take note: All the way up to modern times, (20th Century) you will see a term called Length of Tube or Cannon Length. In many, many of the slightly older books on the subject of artillery, you will find that the author IS NOT referring to the entire length of the gun, but rather, only to the length limited by the distance from the Muzzle Face to the rear of the Base Ring. To get the Entire length of the cannon, they specified Over All Length or Total Length or Length to it's Extremes which included the entire Cascabel AND the Length of Tube.
Artillery terminology; Ain't it fun??
The main use for the Cascabel has always been as an attachment point for tackle when mounting or dismounting a piece from its carriage. Also in naval service the Cascabel has been used extensively as an aid to controlling recoil of the piece by the attachment and use of Breeching hausers or cables attached to it and the ship.
Tracy and Mike